In recent years, for transportation of optical or electronic components or mounting of optical or electronic components on printed boards, each component is often packed with a given sheet, or a pressure-sensitive adhesive tape is often attached to each component, before transfer. In particular, surface protecting films are widely used in the field of optical or electronic components.
In general, such surface protecting films are attached to materials to be protected through a pressure-sensitive adhesive layer provided on a backing film side and used to prevent scratching or staining during the processing or transportation of the materials. For example, a liquid crystal display panel is formed by bonding optical components such as polarizing plates and wave plates to a liquid crystal cell through pressure-sensitive adhesive layers. The surface protecting film is bonded to any of these optical components with the pressure-sensitive adhesive interposed therebetween, so that the component is protected from scratching or staining during processing or transportation. The surface protecting film can be peeled off and removed when becomes unnecessary.
In general, surface protecting films and optical components are made of plastic materials and therefore are highly electrically insulating and can generate static electricity when they are rubbed or peeled off. Therefore, static electricity is generated even when a surface protecting film is peeled off from an optical component such as a polarizing plate, and if a voltage is applied to a liquid crystal in such a state that the generated static electricity still remains, the orientation of the liquid crystal molecule may be degraded, or defects may be produced in the panel.
The presence of static electricity can create a risk of absorbing dust or dirt or a risk of reducing workability. To solve the problem, therefore, surface protecting films undergo various antistatic treatments.
To suppress such electrostatic build-up, there has been disclosed an antistatic method that includes adding a low-molecular-weight surfactant to a pressure-sensitive adhesive and transferring the surfactant from the pressure-sensitive adhesive to the object to be protected (see for example Patent Document 1). In this technique, however, the added low-molecular-weight surfactant easily bleeds out to the surface of the pressure-sensitive adhesive, and if this technique is applied to a surface protecting film, there may be a risk of staining the protected material. Therefore, if a low-molecular-weight-surfactant-containing pressure-sensitive adhesive is applied to an optical component-protecting film, there may be a particular risk of degrading the optical properties of the optical component.
There is also disclosed a pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer containing an antistatic agent (see for example Patent Document 2). In this pressure-sensitive adhesive sheet, an antistatic agent including propylene glycol and an alkali metal salt is added to an acryl-based pressure-sensitive adhesive so that the antistatic agent can be prevented from bleeding out to the surface of the pressure-sensitive adhesive. Even when this pressure-sensitive adhesive sheet is used, however, the phenomenon of bleeding of the antistatic agent is inevitable, so that when the antistatic agent is actually applied to a surface protecting film and aged or subjected to high-temperature treatment, there is a risk of causing a phenomenon in which the surface protecting film is partially lifted up from the protected material due to the bleeding phenomenon.
The film is also required to be easily peelable from an optical component without causing troubles due to the distortion phenomenon associated with peeling, such as disturbances in the orientation of a liquid crystal cell, an increase in the cell gap, and peeling of the optical component from a liquid crystal cell. In recent years, as the screen size of liquid crystal panels has increased, the area of protective films has increased, and the peeling speed has also increased. Therefore, easier peelability has been demanded.
Patent Publication 1: JP-A No. 9-165460
Patent Publication 2: JP-A No. 6-128539